This article from The Conversation, which quickly went viral around the world, argues that those concerned with animal welfare would do better to eat grass-fed beef than bread, because by doing so they would avoid the crushing and poisoning of vast numbers of mice and other small animals in the production of wheat (and presumably other grains or pulses). It is a thought-provoking claim and it might even be right, but the argument seems to have serious holes that I have not seen addressed in any of the comments. (Warning: I have no particular knowledge of farming, so I’m just applying common sense as an alternative.)
The author points out that a lot of land, particularly in Australia, is not fertile enough to be used for any agricultural purpose other than light grazing by livestock or wild animals like kangaroos. In that sense the resulting meat represents a free lunch; if the land were not used for grazing it would not produce any food for humans. Crucially, when the animals are grazed they do not need to eat grain produced on farms that crush and poison mice. But grazing on grass is not all that cows raised for meat generally eat, and I expect it is mostly not what any additional cows we produce will eat. Wikipedia informs us:
Prior to entering a feedlot, cattle spend most of their life grazing on rangeland or on immature fields of grain such as green wheat pasture. Once cattle obtain an entry-level weight, about 650 pounds (300 kg), they are transferred to a feedlot to be fed a specialized diet which consists of corn by-products (derived from ethanol production), barley, and other grains as well as alfalfa. Feeds sometimes contain animal byproducts or cottonseed meal, and minerals. …
In a typical feedlot, a cow’s diet is roughly 95% grain. …
The animal may gain an additional 400 pounds (180 kg) during its 3–4 months in the feedlot. Once cattle are fattened up to their finished weight, the fed cattle are transported to a slaughterhouse.
So there’s one big problem with the argument – not all, but a large share of the meat you eat in beef is just repackaged intensively farmed grains. The rule of thumb in biology is that about 90 per cent of the energy or biomass content is lost as you move up each so called ‘trophic level’ from plants to herbivores to carnivores. If that were the case here then you would need to feed a cow 10kj of grain to get 1kj of meat. Given that the livestock in feedlots are being rapidly stuffed full of calories the efficiency is probably much higher, as they won’t live for long enough to use up much of the energy on their own metabolism. If we guess that in fact the conversion has a 50 per cent efficiency, and that the cow was two thirds edible meat on entering the feedlot, then to get 100kj of meat at the end we needed approximately 100kj of farmed grains. Any benefit then would then only come from the higher protein and fat content of the meat relative to the carbohydrate packed grains that went in.
I think our doubts should go further though. Grazing cattle on land that is not suitable for other agriculture is the low hanging fruit for beef production – if the land does not have other productive uses we don’t give up anything to stick cattle there. For that reason we should expect such land to be used as much as possible for that purpose already. Eventually, as the stock of livestock grows, we should expect to exhaust the flow of foliage growing on this kind of land. Then where are they to go? We could stick them on land that doesn’t supply much grass for them to eat (or grassland where the foliage is already being fully grazed by cows).  But in that case what are the additional cows to eat? The likely answer is the cheapest form of calories we know how to produce: intensively farmed grains like wheat and barley.
Has humanity reached the point where otherwise wasted grassland is fully occupied? Supporting evidence for this is the common claim that higher demand for livestock among a growing Asian middle class is driving up grain prices. If additional cows were largely fed by grass, that wouldn’t be an issue. It is quite possible that if you as an individual switch from bread to beef both more cows will be slaughtered and more mice poisoned as a result of the extra grain needed to support the cows. If the cows were largely grain-fed then it would be many times worse for the mice.
The article has another notable weakness in that it only denominates the number of deaths by protein production. Protein is an important macronutrient but not the only thing we care about getting from our food. Indeed protein deficiency is exceeding rare amongst those wealthy enough to contemplate eating beef. If you denominated the number of lives lost by energy content, then wheat, being mostly carbohydrate, would come out looking a lot better than the 25 mice poisonings to each cow slaughter quoted in the article. The article is also basically irrelevant when judging the treatment of poultry or pigs.
Further, the piece ignores the starvation and predation of small wild animals on land in the absence of intensive agriculture. Probably that isn’t such a large concern as there would be far fewer such animals than there would be mice during plagues, but it deserves consideration. Finally, the quality of life of grazing cows or indeed field mice isn’t mentioned, only their deaths. I am not sure whether such creatures have good or bad lives, but it seems to be a crucial issue for those sincerely concerned about their welfare.
It isn’t possible to say with any certainty that grains are better than beef from an animal welfare point of view, but the effects of our actions here are more complex and need deeper analysis than a short op-ed can provide. The huge popularity of the piece is more likely because it allows those who don’t care or think about animals at all to superficially stick it to vegetarians and claim they were right all along (by pure luck presumably), rather than because its claim really stands up to scrutiny.
UPDATE: This piece attempts to quantify the deaths from different sources of food and produces the opposite conclusion, though the figures for mice killed in harvesting are rubbery and may not apply to the ‘marginal field’.
 We could also graze or place them on highly fertile land that was previously used for crops, but that would be inefficient and unprofitable if you could raise more cows just by having intensively farmed crops and feeding cows the resulting grains elsewhere.